1. Field of the Invention
The present invention relates to a fibrous structure (either of the woven or non-woven type, natural or synthetic etc.). As it is well known, such fibrous structures are used in many industrial applications, such as those given hereafter by way of illustration without any limitation:                hygienic articles (feminine as well as medical/surgical) such as diapers, sanitary napkins, incontinence guards, wipes, wound dressing, face masks and the like;        industrial applications such as for use in isolation products (thermal, electrical) or filtration products, or else floor coverings;        textile products.        
2. Description of Related Art
Depending on the kind of fibrous structures and applications considered, properties of hydrophilicity and/or adhesion will be particularly looked for.
Taking the example of non-woven materials, they are produced from comparatively hydrophobic synthetic fibres such as, for example, fibres of polypropylene or polyethylene which are treated and/or apertured in order to make the materials liquid permeable.
In order to obtain fluid absorbent articles which exhibit good wicking ability, a high total and local fluid uptake capacity, good fluid retaining capacity and a high degree of surface dryness, such articles are usually built up of a plurality of different non-woven fibrous structures having different functions. One major problem when constructing fluid absorbent articles of this kind is, however, that it is difficult to obtain optimal wettability i.e. an optimal degree of hydrophilicity which remains unchanged after the article has been exposed to wetting. Furthermore, it is difficult to obtain stable wetting characteristics in absorbent articles which are stored for an extended period of time.
In WO 91/05108, it has been experimentally shown that there is a connection between increased surface area and increased rate of absorption. The patent application relates to fibres which have been provided with a porous layer attached to the surface of the fibres. The porous layer increases the specific surface of the fibres which implies that absorbent material containing such fibres obtains an improved rate of absorption and wicking ability.
The porous layer is created by impregnating fibrous material with hydrophilic chemicals while the fibres are kept in a dry or in a wet state in the form of dewatered fibre pulp or in the form of an aqueous suspension of fibres, respectively. The treatment may be performed by bringing the fibres into contact with the hydrophilic chemicals for instance by spraying fibres in a formed absorbent layer with a chemical solution or by mixing the chemicals with a fibre suspension wherein the chemicals are added as solids, in a solution, or in any commercially available form.
Regarding fluid permeable cover sheets for use in absorbent articles such as diapers, incontinence guards and sanitary napkins, wherein the cover sheet is intended to be in contact with the body of a user during use, it is important that the cover sheet could stand repeated wettings.
In other words, the cover sheet should remain fluid permeable even after the absorbent article has been exposed to fluid impact several times. Furthermore, it is important that the cover sheet can accept a large amount of fluid during a short interval of time. Another important property of the fluid permeable cover sheet is the ability to exhibit high surface dryness even after having been exposed to several wettings. In order to obtain a cover sheet having the desired properties, it is important that the cover sheet exhibits an optimal, i.e. a desired, degree of hydrophilicity and that the degree of hydrophilicity varies only within a very limited range when the fibrous structure is wetted or when it is subjected to ageing.
As well known to the man skilled in the art, the literature of these fields talk about the properties of “hydrophilicity” or “wettability” of a substrate, or else of “adhesion” of a third body on a substrate and often report measurements of “surface tension”, “contact angle” and “peeling test” to evaluate such properties.
A commonly used method for increasing the wettability of fluid permeable cover sheets for use as cover sheets in absorbent articles, is to treat the material with surfactants. Non-woven materials used as cover sheets for absorbent articles are usually made of synthetic materials which are inherently hydrophobic and which have been treated with surfactants in order to become wettable and readily permeable to fluids. The treatment is usually carried out by coating the hydrophobic material with a surfactant. In order for a material to be fluid wettable, the contact angle between the surface of the material and the fluid must be less than 90°. However, a problem in connection with using cover sheets which have been coated with a surfactant is that such cover sheets exhibit decreasing fluid permeability with repeated wetting. The reason for this is that the applied surfactants are not firmly attached to the surface of the cover material and will be detached from the cover material and solved in body fluid during the first wetting. At subsequent wettings the amount of surfactant which remains on the surface of the cover sheet is therefore considerably reduced, resulting in impaired fluid permeability.
Another problem in relation to the use of articles having surfactant-coated cover sheets is that the surfactant compounds may migrate from the cover sheet to the skin of the user, thereby causing skin irritation.
An additional problem with absorbent articles having cover sheets of this kind is that during storage of the article the surfactants may migrate from the cover sheet into the absorbent structure, resulting in the fluid permeability of the cover sheet being insufficient even at the first fluid impact.
Still another problem with surfactant-coated cover sheets is that the method of applying the surfactant is less attractive from an environmental point of view since the surfactant agent is usually applied to the surface of the material in the form of a solution which, for instance, is sprayed over the surface and causes the surfactant to be emitted into the ambient air.
The present invention provides a fibrous structure of the kind mentioned in the introduction. The fibrous structure exhibits when desired a well defined rate of wetting, i.e. a predetermined degree of hydrophilicity which is substantially unaffected by wetting of the fibrous structure, and/or good properties of adhesion which are substantially unaffected by wetting of the fibrous structure.
Furthermore, with the present invention a fibrous structure is provided, wherein the desired, predetermined degree of hydrophilicity and adhesion properties are maintained even after the structure has been stored for a period of time. Accordingly, the present invention offers a hygienic product having a well defined and controlled course of wetting.
A fibrous structure in accordance with the invention is primarily distinguished by one or several types of polar silicon-containing compounds, being bound to at least one portion of the surface of the fibrous structure by interaction between the surface and the silicon-containing compounds.
As previously mentioned, the fibrous structure according to the invention exhibits a predetermined degree of hydrophilicity and adhesion properties which are substantially unaffected by wetting of the fibrous structure.
In accordance with one embodiment, the silicon-containing compound consists of a compound of the type SiOxHy wherein x preferably is in the range of 1 to 4, and y preferably in the range of 0 to 4.
An advantage with a fibrous structure of this type is that the wetting characteristics of the structure has proved to be substantially constant during wetting and that the fibrous structure is comparatively resistant to ageing.
Without being in anyway limited by the following theoretical explanation of why a hydrophilic surface having polar silicon-containing compounds exhibits a stable hydrophilicity and adhesion properties both after repeated wetting and after ageing of the material structure, one could think that the polar silicon-containing compounds form a kind of clusters which are sufficiently large to inhibit reorientation of polymer chains and, accordingly, the ageing phenomenon. However, the theory is not fully developed and should accordingly not be regarded as being binding to the invention.
As already mentioned, the fibrous structures in accordance with the invention exhibit at least one polar silicon-containing material surface, or portion of a surface. However, it is possible according to the invention to apply silicon-containing compounds to both surfaces of a sheet of material. Further, one or both surfaces of the material may exhibit one or more delimited areas having polar silicon-containing compounds.
According to one aspect of the invention, the fibrous structure comprises one or more non-woven materials.
According to one further aspect of the invention, the fibrous structure of the invention may, for instance, be used as fluid permeable cover sheet for absorbent articles or as a fluid transfer layer between the fluid permeable cover sheet and the absorbent structure in an absorbent article, or for the absorbent structure itself.
According to another aspect of the invention, the fibrous structure of the invention may be used as a liquid absorbing wipe, or as a component in a wipe or the like.
In still another aspect of the invention, the fibrous structure may comprise one or more tissue layers.
As well known to the man skilled in the art, the term “tissue” commonly covers fibrous material based on cellulose or cellulose in combination with synthetic fibres and typically used in the manufacture of household items such as kitchen towels, toilet paper, or napkins, in the manufacture of industrial wipes for absorption of different liquids, or else for the manufacture of layers entering the structure of absorbent articles such as diapers, incontinence guards, sanitary napkins or the like.
The invention additionally concerns an absorbent article such as a diaper, an incontinence guard, a sanitary napkin or the like comprising an absorption body being enclosed between a fluid impermeable cover layer and a fluid permeable cover layer, said article comprising at least one portion comprising a fibrous structure in accordance with the invention.
The fibrous structure may constitute a part or all of the fluid pervious cover layer and/or of a fluid transfer layer positioned between the fluid pervious cover layer and the absorption body.
In a hygienic product for fluid absorption purposes and being made of a plurality of individual layers, fluid transfer between the different layers is of great importance both for the rate of wicking within each individual layer and for the total fluid uptake capacity of the hygienic product. From the above discussion it appears that in fluid absorbent articles of this kind it is very important that all layers of material exhibit a well-defined and stable degree of hydrophilicity which only varies to a very limited extent with wetting and ageing.
According to one of the aspects of the invention, the liquid permeable cover sheet, the fluid transfer layer, and the absorption body have different degrees of hydrophilicity.
According to one of the preferred embodiments of the invention, the fluid transfer layer of the hygiene article comprises a set of several fibrous structures according to the invention, the set of fibrous structures presenting a gradient of degrees of hydrophilicity.
The invention further concerns a hygienic product such as a wipe, a wound dressing or the like, comprising a fibrous structure in accordance with the invention.
The invention further concerns a method for producing a fibrous structure having one or more types of polar silicon-containing compounds bound to at least a portion of a surface of the fibrous structure. The method is primarily distinguished by the fact that the fibrous structure is submitted to an atmosphere comprising excited and unstable species, as-obtained through the application of an electrical discharge to an initial mixture comprising a carrier gas, an oxidant, and at least one type of silicon-containing gaseous compound.
An advantage with a method of this kind, is that it is carried out under dry conditions which implies that the silicon-containing compound does not have to be solved in a solvent before application which means that the method is advantageous from an environmental point of view.
In accordance with a preferred embodiment, the treatment is based on an electrical discharge led in a gaseous mixture, leading to the formation of a plasma.
As well known, a plasma is a gaseous medium containing ions, radicals, electrons, excited and unstable species. It can be obtained through supplying to a gaseous mixture a sufficient amount of energy, at a defined pressure, for example very low pressure or atmospheric pressure.
All the species of the plasma can react between them and/or with the components of the gaseous mixture to create new ions, radicals, and excited species.
When it is carried out at atmospheric pressure with a high voltage electrical signal as energy supply, the plasma is commonly called “corona”.
According to this preferred embodiment, the fibrous structure is therefore submitted to an electrical discharge, in presence of a gaseous mixture comprising at least one type of silicon-containing gaseous compound, oxygen or other oxygen-containing gas, and a carrier gas.
According to another embodiment of the invention, the fibrous structure is submitted to a treatment atmosphere as-obtained in post-discharge of an electrical discharge applied to a gaseous mixture comprising at least one type of silicon-containing gaseous compound, oxygen or other oxygen-containing gas, and a carrier gas (the fibrous structure is here submitted to the treatment atmosphere outside the discharge).
In any case, the unstable and excited species of the atmosphere react with the polymer chains of the surface of the fibrous structure, leading to the formation of radicals of said polymer chains. These radicals can then react with the species present in their vicinity forming this way new bondings and new functional groups on the surface. Functional groups which are relevant to the present invention are polar silicon-containing groups. The functional groups this way introduced on the surface of the material are much more strongly bound to the surface than an active substance which has been applied as a conventional coating.
A method for corona treatment is described in U.S. Pat. No. 5,576,076, U.S. Pat. No. 5,527,629 and U.S. Pat. No. 5,523,124. The gas mixture is based on a carrier gas which usually is nitrogen, a silicon-containing compound and an oxidant. The treatment creates a layer of material having a glassy, hydrophilic surface.
The disclosed method is suitable for use in connection with the invention. However, the invention is not limited to the method described in the above entioned applications, but comprises all types of gas phase treatments in which polar silicon-containing groups are introduced to a surface of a fibrous structure.
In accordance with a preferred embodiment the silicon-containing compound in the gas mixture is a silane compound. Some examples of such compounds are SinHn+2 where n preferably is from 1 to 4, silicon hydroxide, halogenated silanes, alkoxysilane or organosilane. The oxidant is preferably oxygen or other oxygen-containing gases such as, for instance, CO, CO, NO, N2O or NO2. The carrier gas may consist of nitrogen, argon, helium, or a mixture thereof.
According to one of the embodiments of the invention, prior to being treated with the medium comprising unstable and excited species, resulting from the application of an electrical discharge to the gaseous mixture comprising the silicon-containing gaseous compound, an oxidant and a carrier gas, the fibrous structure has been in a first step submitted to a corona discharge under air (surface preparation).